The present invention relates to antennas.
A balun is an electromagnetic device for interfacing a balanced impedance, such as an antenna, with an unbalanced impedance. A balanced impedance may be characterized by a pair of conductors, in the presence of a ground, which support the propagation of balanced signals therethrough. A balanced signal comprises a pair of symmetrical signals, which are equal in magnitude and opposite in phase. In contrast, an unbalanced impedance may be characterized by a first conductor for supporting the propagation of unbalanced (i.e., asymmetrical) signals therethrough with respect to a second conductor (i.e., ground). A balun converts the balanced signals propagating through the balanced impedance to unbalanced signals for propagating through the unbalanced impedance, and vice versa.
Baluns have been employed in various applications. One such application for baluns is in radio frequency (xe2x80x9cRFxe2x80x9d) antenna structures. An antenna structure typically comprises at least one balanced impedancexe2x80x94for radiating and/or capturing electromagnetic energyxe2x80x94coupled with a receiver, transmitter or transceiver by means of an unbalanced impedance. For example, an antenna structure formed from a balanced transmission line may be coupled with the receiver/transmitter/transceiver through an unbalanced transmission line formed from a 50 xcexa9 coaxial cable. Here, a balun is employed as an interface between the balanced transmission line and the 50 xcexa9 coaxial cable.
The inclusion of a balun, however, has a limiting effect on the frequency response of an antenna structure. Antenna structures using baluns typically radiate and/or capture electromagnetic energy within a singular frequency band. By incorporating a balun, multiple antenna structures are required to support a number of frequency bands. For example, a multipurpose wireless device might require a first antenna structure to support a cellular phone (900 MHz) band, a second antenna structure to support a personal communication services (2 GHz) band, and a third antenna structure to support an air-loop communication services band (4 GHz).
The frequency limitations of baluns in antenna structures has now become a problem. Presently, a growing commercial interest exists in providing an increasing number of applications and services to multi-purpose wireless devices. In an effort to minimize the additional antenna structures required for each of these increased services, and thereby reduce the complexity of the overall multi-purpose wireless device, industry has begun to explore a singular antenna structure having a broader frequency response characteristics. Consequently, an alternative to the balun is needed to increase the number of frequency bands supported by a singular antenna structure.
We have invented an antenna structure capable of supporting an increased number of frequency bands. More particularly, we have invented an interface between the balanced impedance and an unbalanced impedance, which does not have the balun""s limiting effect on an antenna structure""s frequency response. In accordance with the present invention, a slotline couples an antenna structure formed from a balanced transmission line, for example, with an unbalanced transmission line, such as a coaxial cable, for example. We have recognized that the frequency response of an antenna structure may broadened by replacing a balun with a slotted transmission line (e.g., slotline).